6(3 INTRODUCTION. 
our experience in respect to many of our native spe- 
cies ; for individuals from various and distant localities, 
especially from those differing in climate, soil, or geo- 
logical structure, are rarely identical in external char- 
acter, but almost always present striking differences in 
the size, thickness, or weight of the shell ; the eleva- 
tion or depression of the spire ; the smoothness or rough- 
ness of the epidermis ; the prominence and number 
of the strise of increase ; the diameter of the open axis 
of the shell, known as the umbilicus ; and in the number 
and magnitude of the tubercles, folds, and other tes- 
taceous deposits which are often formed in the aperture, 
and upon the columella of the shell. These modifica- 
tions are so constant, in some species, that the practised 
eye can thereby distinguish the stations, or rather the 
section of country, from which individuals exhibiting them 
are respectively derived. The same remark is true 
as regards the marine mollusks used for food. The 
dealers recognize the localities of some of them by 
variations which often escape the naturalist ; and they 
sometimes know that distinctions which he considers 
structural and constant, are due only to physical influ- 
ences. It is well known that there is a tendency in 
nature to continue, to successive generations, those mod- 
ifications of form which have, in the first place, been 
introduced by accidental causes, and thus to continue, 
for a time, what have been called permanent varieties. 
But these, it is believed, return, sooner or later, to their 
original type. 
